This invention relates to telephone networks including four-wire circuits forming closed loops which are prone to howling and more particularly to a howling controller, implementing a DSP algorithm, for detecting the presence of howling and canceling the same by introducing an appropriate attenuation to the closed loop.
A positive feedback exists in a closed loop (see FIG. 1) when the following relation (Barkhausen) sets up:
xcex2*G=1,xe2x80x83xe2x80x831)
where xcex2=transfer function on the feedback path, in open loop; and
G=transfer function on the direct path, in open loop.
This relation is equivalent to the following simultaneous conditions:
|xcex2|*|G|=1xe2x80x83xe2x80x83(2)
arg(xcex2)+arg(G)=2*xcfx80, in radiansxe2x80x83xe2x80x83(3)
Such positive feedback usually appears when the closed loop gain is higher than a certain value. The effects of the positive feedback in the baseband (i.e., 300 Hz-3400 Hz) of the telephone circuits are the well-known phenomena of xe2x80x9csingingxe2x80x9d and xe2x80x9chowlingxe2x80x9d. Both of these drastically impair the useful signals, and should, therefore, be under strict control.
A possible definition of howling is the result of a positive feedback developed on telephone network four-wire circuits as a consequence of an imperfect echo cancellation. Analyzing the above noted relation in (1) or the relations in (2) and (3), two possibilities to cancel the positive feedback can be derived:
a) by introduction of an attenuation on the feedback path; or
b) by changing the phase characteristic of the feedback path.
Based on the two above mentioned approaches, the following main methods were used previously:
Phase methods: i) All pass filter with variable group delay characteristics;
ii) Random phase shifting;
iii) Constant frequency shifting with signal re-sampling; and
iv). Two, directional microphones method.
Attenuation methods: i) Variable loss circuit.
A howling signal spectrum changes dynamically, having one or several dominant and non-constant frequencies that can exist practically anywhere in the bandwidth of the useful signal.
The above noted phase methods are based on a characteristic of the human ear of not being sensitive to phase distortions of the audio signal. In the phase methods, the phase of the incoming signal is permanently changed so as the relation in (3) become false.
Method i), as illustrated in the U.S. Pat. No. 5,307,417, dated Apr. 26, 1994, and granted to Takamura et al., is extremely complex and, consequently, makes use of a large number of instructions per second and needs a large amount of memory. Method ii), as illustrated in U.S. Pat. No. 4,449,237, dated May 15, 1984, granted to Stepp et al., and method iii) change the phase characteristic of the feedback path either randomly or deterministically, in an attempt to algebraically compensate the unwanted components of the howling spectrum. The stability margin of the closed loop is slowly increased thus the howling possibility is not completely eliminated. In addition, methods ii) and iii) introduce signal distortion. At least one drawback of method iv), see U.S. Pat. No. 5,323,458, dated Jun. 21, 1994 and granted to Park et al., is that this method assumes a spatial and electrical symmetry, which is not always true; also, the portability of this solution is restricted to only special telephone sets. A general drawback of methods i), ii), iii), and iv) is that all can handle only a limited amount of the original loop gain.
In the attenuation methods, the incoming signal is attenuated so that the relation in (2) become false.
The previous attenuation methods, as illustrated in U.S. Pat. No. 5,379,450, dated Jan. 3, 1995, granted to Hirasawa et al., depend on the signal level, and on the specific mechanical and electrical characteristics of the telephone sets as well. Also, these methods make use of absolute reference levels other than 0 volts, and are characterized by increased complexity and high computational effort. Regarding the actual full-duplex voice switches, as illustrated in the U.S. Pat. No. 5,099,472, dated Mar. 24, 1992, granted to Townsend et al., the attenuation is introduced on the two speech paths permanently, disregarding the presence or absence of howling.
The solution proposed in this patent application involves the introduction of attenuation only when howling is recognized from other signals existing on telephone networks.
Therefore, in accordance with a first aspect of the present invention there is provided a controller for use in a telephone circuit to control howling signals created by positive feedback between receive and transmit paths in the circuit. The controller comprises: detection means to detect an initiation of a howling signal in the circuit; attenuation means to introduce attenuation into one of the receive and transmit paths; and processing means to control the attenuation level as a function of the howling signal.
In accordance with a second aspect of the present invention there is provided a method of controlling a howling signal in a telephone circuit having a receive path and a transmit path, the howling signal being generating by a positive feedback between the paths. The method comprises detecting the onset of a howling signal and introducing attenuation to at least one of the paths to cancel the howling signal.
In accordance with a third aspect of the present invention there is provided an algorithm for controlling a digital signal processor (DSP) howling controller to cancel howling in a telephone circuit having a receive path and a transmit path, the algorithm comprising: detecting the onset of howling by monitoring input samples from the receive path; controllably introducing attenuation to at least one of the receive or transmit paths to thereby reduce the howling signal; and stopping the introduction of attenuation when the howling signal reaches zero.
In a preferred embodiment there is a maximum level of attenuation that can be introduced.
In a further preferred embodiment the algorithm includes prediction logic to verify that the detected signal is in fact a howling signal.